Landau-Ginzburg-Wilson theory is a very powerful tool to study phase transitions and critical phenomena. It takes advantage of the fact that the low energy physics is dominated by a length scale which is diverging at a critical point, namely, the correlation length of order-parameter fluctuations. So one can derive an effective theory in terms of a single order-parameter field by integrating out all the other degrees of freedom with smaller length scales. However, this theory will breakdown if there are soft (massless) modes other than order-parameter fluctuations at criticality. These extra soft modes lead to power law behaviours in various physical correlation functions even far away from the critical point. Distinct from critical scale invariance, this is called generic scale invariance (GSI). Following Ref [1], I will talk about two major mechanisms that leads to GSI. Then I will use an example to discuss how GSI influences the critical behaviour in classical systems.